The present invention relates generally to a method and apparatus for measuring performance of an optical element, and more particularly to a measuring method and apparatus for measuring a wave front of a projection optical system that transfers a pattern on a mask onto an object, etc. The present invention also relates to an exposure method and apparatus suing such a measuring method and apparatus. The inventive measuring method and apparatus are suitable, for example, for measurements that use as measuring light synchrotron radiation, such as a synchrotron ring, an undulator, etc.
A projection exposure apparatus is used to transfer a pattern on a mask (or a reticle) onto an object to be exposed in manufacturing semiconductor devices, etc. in the lithography process. This exposure apparatus is required to transfer the pattern on the reticle onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having good imaging performance and reduced aberration. In particular, due to the recent demands for finer processing of semiconductor devices, a transfered pattern is sensitive to the aberration of the optical system. Therefore, there is a demand to measure the wave front aberration of the projection optical system with high precision.
FIG. 6 shows an optical path of a conventional lens performance measuring apparatus 100. In FIG. 6, 101 denotes a target optical element or optical system, such as a projection optical system. 102 denotes an object surface of the target lens 101. 103 denotes an image surface. 109 denotes a condenser lens, which has a final surface as a reference surface for reflecting part of incident light. 108 denotes a mirror for deflecting the measuring light. 105, 106 and 107 denote stages that are mounted with the condenser lens 109 and the mirror 108 and move in X, Y and Z directions, respectively. 113 denotes a spherical mirror, and its center of the radius of curvature approximately accords with the object surface 102. 110, 111 and 112 denote stages that are mounted with the spherical mirror 113 and move in X, Y and Z directions, respectively. 104 denotes an interferometer body, which houses a laser light source (not shown), a lens (not shown), a collimetor lens (not shown), a beam splitter (not shown), an interferometer condenser lens (not shown), a camera (not shown), etc.
According to the above structure, a collimated ray emitted from the interferometer body 104 is reflected on the spherical mirror 109's final surface and incident as interference light upon the interferometer body 104, forming interference fringes on the camera (not shown). The wave front aberration of the target optical system 101 is calculated from the obtained interference fringes. In order to measure plural positions on the image surface 103 of the target optical system 101, the stages 105, 106 and 107 that install the condenser lens 109 may move to a predetermined position, and the stages 111, 112 and 113 that install the spherical mirror 113 may move to a corresponding position. Such an apparatus is disclosed, for example, in Japanese Patent Application, Publication No. 9-98589.
The conventional measuring apparatus that uses the ultraviolet (“UV”) light as measuring light can easily reflect the light using a mirror and thus easily measure plural positions on the image surface 103. On the other hand, due to the demand for the fine processing of the semiconductor device, the practical implementation of a reduction projection exposure apparatus that utilizes the extreme ultraviolet (EUV) light having a wavelength between 10 and 15 nm, shorter than the UV light is now promoted. It is conceivable that an interference measurement of an EUV optical system utilizes an intensifier EUV light source, such as an undulator light source inserted into an electron accumulation ring. Since the electron accumulation ring should maintain the inside ultra high vacuum (“UHV”), the optical element is provided in the UHV and free orthogonal driving of the stages 105 to 107 shown in FIG. 6 becomes difficult. In particular, it becomes difficult to displace the measuring light in a direction perpendicular to the optical-axis direction. As a result, it becomes difficult to measure plural positions on the image surface, or a necessary area of the target optical system.